A number of polymers and polymer blend compositions have been developed over the years for fabricated articles such as grips, handles, push buttons, auto arm rests, cable jackets, decorative trim profiles, and extruded sheet laminates for tarpaulins, seat covers, and automotive instrument panel skins. Plasticized polyvinyl chloride (P-PVC), thermoplastic polypropylene and/or polyethylene olefin elastomer blends (TPO), and polyurethanes (PUR) have been extensively employed for these applications. Each of these blends, due to inherent composition shortcomings, have shown deficiencies as to weatherability, mar resistance, and recyclability in these applications.
The current design technology for instrument panels involves foam-in-place or foam placement techniques, in which a P-PVC or PUR or TPO skin is vacuum formed or slush molded, a separate structural substrate is molded of a styrenic or olefin resin, and a polyurethane or olefin foam is injected or positioned into the gap between the skin and the instrument panel structure.
The product primarily utilized within the automotive industry for instrument panel skins is P-PVC. Many attempts have been made in the past with only limited success, and intensive efforts still continue, to displace plasticized P-PVC. The requirements for this application include resistance to abrasion wear, heat deformation, and ultraviolet degradation, as well as low volatile migration and the capability of replicating desired finishes with long-term grain retention. Additionally, compositions for components such as instrument panel cover stock must be colorable and of a gloss level so as not to cause glare or reflectance which will detract from the vision of the driver. To define an acceptable surface, the industry uses both a visual inspection and a quantitative measure of the reflective gloss of an extruded sheet. Moreover, the instrument panel skin material should be compatible with the substrate, i.e., foam and/or structural components of the instrument panel, to enhance recyclability.
The predominant instrument panel skin material, P-PVC, possesses the recognized disadvantage of volatile plasticizer migration. Instrument panel skins made from PVC must be plasticized to provide appropriate softness. However, plasticizers are somewhat volatile, due to their low molecular weight and, as the part ages within the interior of the car, the volatile plasticizer deposits on the windshield and other interior surfaces. This deposit is commonly referred to as "fogging", and as such is found to be undesirable. In addition, migration of the plasticizer from the P-PVC leads to long-term embrittlement of the material and loss of function. Therefore, in many instances the P-PVC skin is coated to control gloss levels, adding additional expense to the manufacturing cost. Its most notable disadvantage is that of disposal and recycling. Typically, it is not compatible with the foam substrate or structural component materials of the instrument panels, and as such is not conducive to recycling. This normally bonded structure of dissimilar materials necessitates incineration, resulting in the production of hydrogen chloride and heavy metals as a by product, or the necessity of land filling.
PUR, a small but growing material segment for instrument panel skins, overcomes the deficiency of plasticizer migration, but is expensive, in many instances still requires an over coat for gloss control, and is not compatible with most of the materials of construction for the instrument panel, resulting in difficulty in recycling.
A TPO, in this case consisting of a compounded or reactor blend of polypropylene and olefin rubber, which addresses the issues of P-PVC and PUR with regard to plasticizer migration, cost, and recycling, brings with it a weakness in mar resistance. The current technology for TPO instrument panel skins utilizes a coating to achieve the required scuff and mar resistance properties. Additionally, an adhesion promoter is required to allow the coating to bond properly, which makes it a costly and time-consuming requirement to achieve adequate mar resistance. These types of blends are disclosed in U.S. Pat. No. 4,871,810 and U.S. Pat. No. 4,968,752, but the different blends of these patents are either too hard or poor in abrasion resistance compared to the blends of the present invention. U.S. Pat. No. 5,206,294 discloses a TPO material for instrument panel skins of allegedly improved abrasion resistance, but has had only limited commercial success due to inadequate performance in this regard.
There obviously remains a need in the market for a thermoplastic material which offers the recycling benefits provided by an olefin, but with the mar resistance, feel, and useful service temperature range of P-PVC and PUR resins, and which can provide a uniform grain definition and controlled low gloss appearance without the need for applied coatings. Additionally, an ability to co-extrude this material in combination with other solid and foamed alpha-olefins, without requiring adhesives or tie layers, where a low cost substrate layer can be balanced with an improved functional top layer to minimize cost or maximize physical properties, would offer a broad latitude in product design flexibility and manufacture. Such a material would provide significant design, manufacture, and environmental benefits for automotive interior applications, especially for instrument panel skins. The provision of the blend or alloy of the present invention goes far toward satisfying this obvious need and its many-faceted requirements.